In process metrology, radiometric measurement systems comprising scintillation counters or scintillation detectors for the measurement of radiation are often used for measuring process variables or material properties, for example for filling level measurement, for moisture measurement, for density measurement etc.
Scintillation counters serve, for example, for determining the spectrum of ionizing radiation, that is to say for determining the intensity as a function of the energy of the ionizing radiation, wherein a scintillation counter comprises a scintillator, which is excited during the passage of radiation in the form of high-energy charged particles or photons and emits the excitation energy again in the form of light pulses (usually in the UV or visible range), this being referred to as scintillation.
The light pulses generated in this way are converted into electrical signals by means of a suitable optical sensor and amplified. Such a sensor is typically a photomultiplier or a photodiode. The optical sensor outputs pulses, wherein a number of pulses per unit time, that is to say the counting rate, is a measure of the intensity of the radiation and a pulse height or pulse amplitude (more precisely an integral over the temporal profile of the pulse) is a measure of the energy of the radiation.
In order to measure a process variable in the form of a filling level in a container, by way of example, the radiation from a working radiation source can be applied to the container on one side and an intensity of the radiation can be measured on the opposite side to the working radiation source by means of a scintillation counter. The measured intensity or counting rate is dependent on the filling level of the container since, when material to be measured is present in the beam path, part of the radiation is absorbed by the material to be measured, that is to say that the filling level can be determined in a manner dependent on the measured counting rate.
The measured counting rate is subject to drift effects, however, which can be caused for example by temperature fluctuations, ageing effects of the scintillator and/or of the photomultiplier and/or drift effects in evaluation electronics. These drift effects lead to measurement errors which can make it impossible to determine the process variable reliably.
A method for the automatic drift compensation of a scintillation counter is described in U.S. Pat. No. 3,800,143, for example. For the purpose of drift compensation, a first counting rate of pulses whose energy level or pulse height lies above a first predeterminable threshold is determined, and a second counting rate of pulses whose energy level or pulse height lies above a second predeterminable threshold is determined. The pulse height depends not only on the energy of the observed particle or photon but also on a total gain or a total gain factor of the scintillation counter, which determines the pulse height for a given energy of the particle or photon impinging on the scintillator. The gain is determined, inter alia, by a level of a high voltage applied to the photomultiplier and by the gain of evaluation electronics that generate from the signal output by the photomultiplier, by means of analogue conditioning, a signal in a level range suitable, for example, for digital evaluation by means of a comparator and a microprocessor connected downstream. The change in the total gain, for example as a result of a change in the high voltage supplying the photomultiplier, brings about a change in the spectrum measured by the scintillation counter in the energy direction, as a result of which a counting rate ratio between the first counting rate and the second counting rate changes. It has been found that, with a suitable choice of the thresholds, the counting rate ratio is substantially determined only by the working radiation source used and, in the case of a filling level measurement, for example, does not depend significantly on the filling level. Consequently, a drift compensation can be obtained by the total gain being adjusted in tracking fashion in such a way that the counting rate ratio corresponds to a predetermined function, in particular remains constant.
This type of drift compensation presupposes that the gain is adjustable in the course of a compensation process in such a way that the desired counting rate ratio is produced. However, since the working radiation source is used for drift compensation, said source also serving for measuring the process variable, the actual radiation intensity at the scintillator is dependent on whether media, and if appropriate which media, are situated in a radiation path. Therefore, the drift compensation is not subject to exactly predictable boundary conditions. If, by way of example, an excessively low counting rate is determined at the beginning of a drift compensation on account of highly absorbent media in the radiation path, the total gain is increased, for example as a result of the high voltage of the photomultiplier being increased. If the desired ratio is not established, the total gain is increased further, as a result of which noise pulses possibly also contribute to the measured counting rate. This, in turn, can lead to a corruption of a measured process variable, which is impermissible in the case of systems having a functional safety prescribed by the standards, for example.
If an element is referred to as being coupled or connected to another element, the element can be coupled or connected directly to the other element or interposed elements can be present. However, if an element is referred to as being directly coupled or directly connected to another element, no interposed elements are present.